762 FLORAL ZONES OF THE POTTSVILLE FORMATION. 



alono- the border of the field, in the scallops at the west extremitT of 

 the Wiconisfo Basin and in the angle of the "fish tail"' west of Tre- 

 mont. while it also appears in Sharp Mountain itself east of Middle- 

 port, and in the Sunmiit Hill district. The occurrence of another lobe 

 of the field along the apparently rigid Sharp Mountain, in the region 

 of Ijorberry Gap, seems only to have been escaped by an overthrust 

 faidt of the basal portion of the Coal Measures, as will be shown in a 

 later portion of this report. 



DESCRIPTIOX OF THE POTTSVILI.E FORMATIOX IX THE 



TYPE REGIOX. 



As has already been remarked, the Pottsville formation is, in the type 

 region, composed chieflj'^ of massive siliceous conglomerates. It will 

 be seen later, in the course of a comparison of various sections, that 

 this topographicalh'^ conspicuous formation, which constitutes the 

 floor of the Coal Measures, comprises a series of ponderous conglom- 

 erates, which are more variable in color, composition, and assortment 

 in the lower part, and more quartzose, dense, and light colored near 

 the top. These conglomerates alternate near the base with washes of 

 purple and olive mud or soft, greenish sandstone, and in the higher 

 portion with thin beds of arenaceous shale, and arc interspersed with 

 a number of carbonaceous beds, some of which, in portions of the 

 field, are workable over considerable areas, their product being the 

 most valuable of the anthracite coals. 



The formation, as a whole, varies greatly in thickness, the maximum 

 of a little more than 1,200 feet being reached in the vicinity and to the 

 west of the type section, east of which it thins remarkably. That it 

 thins toward the west in the Southern field itself has more recently 

 been doubted. It is clear, however, that the relative thickness of its 

 divisions is quite different in some of the sections, if the total depth 

 remains the same. The sandstones, like the coals, are extremely vari- 

 al)le even within short distances.^ Northwestward the formation thins 

 rapidly in the anthracite regions, its development being about 850 feet 

 in the Shamokin district of the Western Middle field, or an average 

 of about 350 feet in the Eastern Middle field, while it is recorded as 

 averaging 2-25 feet in the Northern Anthracite field. 



In the Southern Anthracite field, the formation is apparently con- 

 formable with the Mauch Chunk shales, while the line of separation 

 bet\veen it and the superimposed Coal Measures, which are also highly 

 arenaceous, abounding in conglomerates, has for convenience been 

 drawn at the lowest workable coal in the type region. 



1 Compare columnar sections on columnar-section sheet xi, Pt. IV B of the Atlas of the Southern 

 Anthracite Field. 



